Mullard light sensitive transistor known as a 'Phototransistor'. As these were more expensive than ordinary transistors, it was found that an OC71 transistor (much cheaper) was also sensitive to light if the paint was remove from its surface. Mullard later changed the jelly surrounding the germanium element inside probably to stop this being possible.

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The so called "Gunge" in the transistor encapsulation is Silicon Grease which provides some heat conduction from the transistor junctions.
To improve the heat conduction a mixture of silicon oil and aluminium oxide was used - this also improved the performance by reducing effects of light reaching the sensitive transistor junction. To give mechanical protection to the delicate wire connections of the Indium pellets which formed the transistor Collector and Emitter junctions the bare assembly could be dipped into blue Silicon "Bouncing" Putty before being placed into the alundum filled glass bulb which had been centrifuged to expelled any air pockets which might reduce heat conduction. The Black finish was a cellulose dope onto which the Type Number could be transfer printed. This lacquer was very flammable and was applied by a rotary machine housed in a sealed flameproof room which in the event of a fire could be automatically flooded with CO2 - this made one quite nervous when it was necessary to work on this paint machine as there was only 10 seconds to exit the room before drowning in CO2.

.......... John Paine, London UK, 19th of August 2015

The OCP71 was a Photo Transistor being a clear enveloped version of OC71 germanium transistor. It could be used as a photo diode.
The original OCP71 germanium photo-transistor (filled with clear gunge) cost an obscene amount of hard earned cash - twelve shillings and six pence whereas the OC71 typically cost four shillings and six pence (still a lot when pocket money was earned rather than just given). Since the only discernible difference was the thin black paint of the outer envelope and the first OC71 transistors had clear gunge, so the black paint was often scraped off to achieve a lower cost equivalent photo-transistor thus saving eight shillings (a fortune to a young lad at that time). Subsequently when this practise was noticed by Mullard, the filling in the OC71 was changed to the opaque white or blue one to prevent such cost saving practises. I remember thinking "what a bunch of old meanies" at the time. I even tried cutting the glass envelope off and scraping the opaque gunge out with a pin head and then taping the glass envelope back on to achieve the lower cost photo-transistor objective. It was really not very good but I still have this failed experiment, never being able to bring myself to discard this expensive experimental component.
Once on an earlier clear OC71 the base lead broke off (metal fatigue) at the glass envelope and then ages were spent scraping away at the glass and finally using a modified soldering iron and some thin tinned copper wire, a new base connection was soldered onto the exposed stub. This transistor was then used successfully with extreme care for many years and I still have it to remind me of the expensive hobby days.

.......... John Goldthorp, Cambridge UK, 13th of February 2012

I remember many hours of work trying to remove hum from an amplifier. Eventually it turned out to be an OC71 transistor on which the paint had flaked and it was picking up the fluorescent lights, acting just like an OCP71. Perhaps that is why they later used an opaque grease? .......... Wheelyjon, Woodbridge, UK, 5th of March 2011